MidiKeys is a screen representation of a piano keyboard. 

It can play through the internal speaker in the PC, or it can play
sounds using the Windows Multimedia Extensions if you have a sound card
or an external MIDI sound generator.

MidiKeys is very useful for setting up the MIDI Mapper, it lets you
audition the sounds on a sound card or MIDI sound module, and it can be
fun to just bang on little melodies.